Several years ago a TV magazine show --can't remember which one-- actually planted one in a bike in NYC to see how long it would take for it to be stolen. It had a chain lock on it and a hidden camera was set up to film the whole process. This was in the middle of a week day in downtown Manhattan. The way I remember it is that it took less than an hour before a messenger came by, looked the bike over, locked his bike and then took a bolt cutter from his shoulder bag. Cut the chain and rode off. The TV crew tracked him to the place where he sold it.
It seems they embedded the item in the seat post So there are devices available small enough for a bike, but how cheap they might be, I don't know.
What irritated me about the whole story was that the TV crew did not have the guy arrested. They let him go after he promised not to do it again.
MY question is: if you are that paranoid, why would you leave an expensive bike unattended. If you are going someplace that you know you will get off the bike, ride an old piece of junk or buy the best lock and cable available.